Cathode ray storage tube having target with photochromic memory device

ABSTRACT

A non-viewing storage cathode ray tube having write, read and erase functions utilizes a target comprising photochromic material. One side of the target is covered with a phosphor layer which emits ultraviolet light upon irradiation by a writing and/or a reading electron beam. The other side of the photochromic target is covered by a layer of radiation sensitive material. The material may comprise a phosphor which emits red light upon irradiation by a reading or erasing electron beam. In the alternative, the radiation sensitive material may comprise a photoconductive layer sensing the transmission of light through the photochromic glass.

United States Patent [191 Steinberg 1 June 5, 1973 [54] CATHODE RAY STORAGE TUBE HAVING TARGET WITH PHOTOCHROMIC MEMORY DEVICE 175] ihvefiidr'; ljavid Ronald Steinberg, Raleigh,

N.C. [73] Assignee: Corning Glass Works,Corning,N.Y. ['22 Filed: July 30,1971

[21 Appl.No.: 167,606

[52] US. Cl. ..3l3/65 R, l78/DIG. 31, 313/69 R, 313/70 R, 313/92 R, 3l5/8.6, 315/13 ST,

350/160 P [51] Int. Cl ..H01j 29/18, H0lj 31/62 [58] Field of Search ..313/65 R, 65 A, 65 T,

313/69 R, 70 R, 92 R, 92 LF, 108 R, 108 A; 315/85, 8.6,13 ST; 350/160 P; 178/7.5 D,

DIG, 31

[56] References Cited UNITED STATES PATENTS 3,242,367 3/1966 Szegho ..3l3/65 A X 3,328,777 6/1967 Hart ..315/8.6 X 3,400,214 9/1968 Hamann ..l78/7.85 X 3,408,531 10/1968 Goetze et a] ..3l3/92 R X Primary ExaminerRoy Lake Assistant ExaminerSiegfried H. Grimm Attorney-Clarence R. Patty, .lr., Walter Zebrowski, and Woodcock, Washburn, Kurtz & Mackiewicz 57 ABSTRACT A non-viewing storage cathode ray tube having write, read and erase functions utilizes a target comprising photochromic material. One side of the target is covered with a phosphor layer which emits ultraviolet light upon irradiation by a writing and/or a reading electron beam. The other side of the photochromic target is covered by a layer of radiation sensitive material. The material may comprise a phosphor which emits red light upon irradiation by a reading or erasing electron beam. In the alternative, the radiation sensitive material may comprise a photoconductive layer sensing the transmission of light through the photochromic glass.

14 Claims, 4 Drawing Figures PATENTEUJUH 51915 saw 1 or 2 i? 0 OJ) 0 04m 0 Of) wf NN CATHODE RAY STORAGE TUBE HAVING TARGET WITH PHOTOCHROMIC MEMORY DEVICE BACKGROUND OF THE INVENTION This invention relates to non-viewing storage cathode ray tubes utilizing photochromic material as a storage medium.

Such a tube stores information by the illumination and resulting darkening of the photochromic material at selected locations of the target. The illumination is often accomplished by coating one side of the photochromic glass with a phosphor layer which is then irradiated by an electron beam at selected locations of the target. The'light emitted by the irradiated phosphor darkens or renders substantially opaque the photochromic material at the selected locations.

In order to retrieve the stored information, the darkened locations of the photochromic material are determined by illuminating the photochromic material and detecting light transmission through the photochromic material. In some cases, the same phosphor layer which was irradiated for storing information in the photochromic material is irradiated by an electron beam of lesser intensity to produce light emission from the phosphor. The intensity of light transmitted through the photochromic material which is substantially lower where the photochromic material is darkened may then be detected by a suitable photon detector located on the other side of the target. In other cases, a phosphor layer has been provided on the other side of the photochromic glass which is irradiated by another electron beam and light transmission in the opposite direction through the photochromic material is detected.

The information stored in the tube is cleared by bleaching the photochromic glass. This has been accomplished by high intensity illumination of the photochromic glass or by heating the photochromic glass.

Read-out losses have presented a problem in prior art photochromic storage tubes. In those tubes where a photon detector is utilized externally of the tube envelope, detection efficiency may be rather low due to light attenuation by the tube envelope. In this connection, reference is made to U.S. Pat. No. 3,148,281 Fyler, wherein direct detection of photons emitted from an electron-beam-irradiated phosphor layer is relied upon. Any effort to compensate for the attenuation of light due to the tube envelope by utilizing a reading electron beam of higher intensity may result in an illumination of the photochromic material which erases or bleaches the information stored. This is undesirable since such a system cannot retain information after read-out. Actually, not only would erasing or bleaching occur, but the higher intensity may result in darkening of unwritten areas in the material. This would have the same effect of reducing the signal-to-noise ratio that erasing or bleaching would have.

The erasing itself has presented a problem in the prior art. In many instances, it is desirable to provide selective erasure of the photochromic glass. In the system disclosed in the Fyler patent, erasure is provided by heating the photochromic glass so as to bleach the darkened areas. There is no selective erasure.

Many of the prior art photochromic storage tubes can be plagued by electron beam spot position errors of the type which lead to raster distortion. In prior art systems having a writing electron beam which irradiates one side of the photochromic glass target and a reading electron beam which irradiates the other side of the target, such as that shown in U.S. Pat. NO. 3,400,214 Hamann, any spot position error in the writing electron beam will result in a faulty information retrieval by the reading electron beam and vice versa. In systems of this type, spot position errors are additive rather than self-cancelling.

SUMMARY OF THE INVENTION It is one object of this invention to provide an improved photochromic storage tube system having a high read-out efficiency.

In accordance with this object, a photochromic storage tube is provided comprising photochromic'material, a layer of ultraviolet light emitting phosphor on one side of the photochromic material and a layer of light sensitive material on the other side of the photo chromic material. When the writing electron beam is moved across the target while irradiating the phosphor layer, the photochromic material is darkened at selective locations. When the reading electron beam is moved across the target, the light sensitive layer senses the light transmission through the photochromic material as affected by the darkening due to the writing electron beam. The improved reading efficiency is achieved where the radiation sensitive layer comprises a red light emitting phosphor. When the reading electron beam irradiates the ultraviolet light emitting phosphor, the resulting ultraviolet light transmission through the photochromic glass illuminates the red light emitting phosphor. Since the red light emitting phosphor characteristically luminesces with a typically high photon conversion efficiency, high efiiciency reading may be obtained. Furthermore, red photons will penetrate the tube envelope with very little attenuation thereby enhancing efficiency. The improved reading efficiency may also be obtained by direct detection of the ultraviolet photons emitted by the ultraviolet emitting phosphor where the radiation sensitive layer is photoconductive. As the reading electron beam irradiates the photoconductive layer, the photoconductive layer may be illuminated by an independent light source through the selectively darkened photochromic material. The current flow resulting from irradiation by the reading electron beam as affected by the illumination of the photoconductive material through the photochromic glass may then be detected.

It is another object of this invention to provide an improved photochromic storage tube system having a selective erasure capability.

In accordance with this object, the target comprises a red phosphor layer. The red phosphor which may be applied directly to the photochromic material or to an other light sensitive layer is then irradiated by an erasing electron beam at selected locations. The red light emitted by the phosphor upon irradiation selectively bleaches the photochromic material.

It is a further object of this invention to provide an improved photochromic storage tube system wherein electron beam spot errors may be tolerated.

In accordance with this object, the writing and reading electron beams of the device, whether irradiating one or both sides of the target, are deflected by the same electromagnetic or electrostatic fields. Since the same fields act upon both the writing electron beam and the reading electron beam, any electron beam spot BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the invention will be more fully understood from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a simplified sectional view of a photochromic storage tube system embodying the invention;

FIG. 2 is an enlarged view of a portion of the photochromic target in FIG. 1;

FIG. 3 is a simplified sectional view of another photochromic storage tube system embodying the invention; and

FIG. 4 is an enlarged view of a portion of the photochromic target in FIG. 3. 7

DESCRIPTION OF THE PREFERRED EMBODIMENTS An information storage and retrieval system comprising a non-viewing photochromic storage cathode ray tube 10 is shown in FIG. I. The tube 10 comprises a target 12 including a photochromic material, a first electron beam source or gun 14 for generating writing and reading electron beams, and a second electron beam source or gun 16 for generating an erasing electron beam. When the tube 10 is operated in the storage mode, a writing electron beam is moved across the target 12 by a first deflection and focus means 18. Similarly, when the tube 10 is operated in the retrieval mode, the reading electron beam generated by the source 14 is moved across the target 12 by the deflection and focus means 18. When the tube 10 is operated in the erasing mode, an erasing electron beam from the source 16 is moved across the target 12 by a second deflection and focus means 19.

As shown in FIG. I and in somewhat further detail in FIG. 2, the target 12 comprises photochromic material 20 such as photochromic glass in thin layer or thin bulk form or photochromic fiber optics lying between a first layer of phosphor 22 and a layer 24 of light sensitive material comprising another phosphor. A thin layer 26 of conductive material such as aluminum covers the first phosphor layer 22 and a layer 28 of transparent or semitransparent conductive material covers the second phosphor layer 24. The layer 22 comprises an appropriate phosphor so as to emit ultraviolet light upon irradiation by an electron beam while the layer 24 comprises a suitable phosphor so as to emit red light in response to ultraviolet light emission from the layer 22. It should be understood that the conductive layer 28 may be eliminated in certain cases. If the proper accelerating voltage is used, no charging of the screen will result and a conductive layer is not necessary.

When the tube 10 is operated in the information storage or writing mode, the ultraviolet light emitting phosphor of the layer 22 is irradiated by the writing electron beam from the source 14. The resulting ultraviolet light emission from the layer 22 darkens the photochromic material 20 at those locations of the target 12 which the writing electron beam irradiates. The time required to darken the selected location of the photochromic material will depend on the efficiency of the phosphor in the layer 22, the sensitivity of the photochromic material 20, the electron beam intensity, and other factors well known in the art.

When the tube 10 is operated in an information retrieval or reading mode, the reading electron beam, having a substantially lower intensity than the writing electron beam, penetrates the aluminum layer 26 and irradiates the phosphor layer 22 resulting in ultraviolet light emission therefrom. The ultraviolet light which is transmitted through the photochromic material 20 is then sensed by the red phosphor layer 24 which emits red photons when irradiated by ultraviolet photons. The resulting red photons which are emitted from the red phosphor layer 24 are then detected by a suitable reading means in the form of a photon detector 30 located behind a window 32 in the envelope 34 of the tube 10.

Due to the darkening of the photochromic material 20 at selective locations, the ultraviolet light transmission through the photochromic material 20 varies. Where the photochromic material 20 has been darkened at a selected location of the target 12 by the writing electron beam during the writing mode, irradiation of the target at the same location by the reading electron beam will produce relatively few if any red photons for detection at the photon detector 30. However, where the photochromic material 20 has not been darkened at a selected location of the target 12 by the writing electron beam, the photochromic material 20 will permit ultraviolet photon transmission to the red phosphor layer 24 with resulting emission of red photons for detection at the photon detector 30. Since the phosphors of the layer 24 will luminesce redphotons with an intensity inversely proportional to the level of darkening in the photochromic material 20, a signal is obtained from the detector 30 which is inversely proportional to the level of darkening of the photochromic material. However, the intensity of the radiation from the red light emitting phosphor in the layer 24 is not high enough to cause significant bleaching of the photochromic materials. Thus, information can be retrieved from the tube 10 without erasing the information. The output of the detector 30 may then be applied to a computer memory or a display cathode ray tube.

In the erasing mode, the erasing electron beam penetrates the transparent conductive layer 28 and irradiates the phosphor layer 24. As a result, the red light emitting phosphor of the layer 24 luminesces red light which bleaches the photochromic material 20. Since the position of the erasing electron beam from the source 16 may be controlled by the deflection means 19, erasing may be selective, that is, limited to selected locations on the target. It is thereby possible to erase any location of the target 12 without erasing surrounding locations thereof. As in the case of writing, the time for erasing darkened locations of the photochromic material depends on phosphor efficiency, the photochromic material, and other factors well known in the art.

The previously described information storage and retrieval system comprising the tube 10 is particularly well adapted to storage and retrieval of binary encoded data although other data in other forms may be utilized. For binary encoded data, a darkened location of the photochromic material 20 may correspond to a 1" while an undarkened location corresponds to a 0. With the system described, storage densities in excess of 10 bits per square inch of photochromic material can be achieved. Thus it will be appreciated that high capacity storage is provided in a relatively small device characterized by a single electron beam source.

It will also be appreciated that the system is substantially immune to electron beam spot errors of the type which lead to raster distortion during information storage and retrieval. As the writing and reading electron beams are either digitally indexed or scanned across the target from location to location, any location error in the writing electron beam will be duplicated in the reading electron beam thereby cancelling the effect of these errors since deflection and focusing is achieved by the deflection and focusing means 18 common to both writing and reading electron beams. Note further that modulation of the writing electron beam and the reading electron beam is facilitated by the use of a single electron beam source.

In the foregoing embodiment, very little reading light is lost passing through the red light emitting phosphor layer 24 since this layer luminesces with a typically high photon conversion efficiency in the range of 80 to 100 percent. Furthermore, the red light is not significantly attentuated in passing through the envelope 34, and accordingly, no special optical window need be provided. This means that reading of the stored information may be accomplished with reduced electron beam power, of the order of one one-hundredth the power of the writing electron beam. This precludes any significant darkening of the photochromic material during reading and further precludes erasing or bleaching of photochromic material since the intensity of light required for bleaching is an order of magnitude higher than that required for darkening. The embodiment is also particularly desirable since a detector such as a photodiode or a photomultiplier is readily available for the red part of the optical spectrum.

Although an ultraviolet photon sensitive red phosphor layer is a particularly desirable means for reading or retrieving stored information from the tube 10, a red phosphor may be provided for the layer 24 which is insensitive to ultraviolet photons emitted by the ultraviolet phosphor layer 22. In such a tube, reading may be accomplished by direct detection of ultraviolet photons which penetrate the phosphor layer 24 and the transparent or semitransparent conductive layer 28 and are detected by an ultraviolet photon detector 30. In such an embodiment, a special ultraviolet photon window 32 must be provided in the envelope 34. In the alternative, the ultraviolet photon detector may be located within the envelope 34 eliminating the need for a special window 32.

As a further variation to the tube 10, the layer 24 may comprise another light sensitive material such as a photoconductive or photo-emissive material which in itself serves as a reading detector. In such a tube, the

detector 30 could be eliminated. Note that a red phosphor layer overlying the photoconductive or photoemissive layer may still be employed for purposes of selective target erasure. A target employing a photoconductive layer will now be described with reference to FIGS. 3 and 4.

As shown in FIG. 3, the information storage and retrieval system comprises a tube 50 having a photochromic target 52, a writing electron beam source or gun 54 and a reading electron beam source or gun 56. By cating the writing electron beam source 54 and the reading electron beam source 56 in substantially the same plane as the target 52 and providing a 45 at a potential of V, and the retarding electrode 58 which is maintained at a potential V the x deflection of the writing and reading electron beams may be controlled. The two reading beam and two writing beam trajectories shown in phantom represent the application of different electric fields between the target 52 and the retarding electrode 58. The y deflection is provided by conventional electrostatic deflection plates 59 located near the electron beam sources 54 and 56 or by other suitable deflection means. Other aspects of a 45 deflection system which are now well understood in the art are described in an article by H. Heynisch, A Cylindrical Coding Tube for 8-Digit Code, Proceedings of the IEEE, November 1963 which is incorporated herein by reference.

In addition to the elimination of electron beam spot position errors, the location of the electron beam sources 54 and 56 substantially in the plane of the target 52 and at one edge thereof permits the full view of the target 52 thereby making optical reading and bleaching possible.

As best shown in FIG. 4, the target 52 comprises an ultraviolet light emitting phosphor layer 60, a light sensitive layer 62 comprising a photoconductive material, and a photochromic material 64 therebetween. Once again, the photochromic material may be in the form of photochromic glass or photochromic fiber optics.

A semi-transparent conductive layer 66 covering the phosphor layer 60 is also provided along with a transparent signal electrode 68 sandwiched between the photochromic material 64 and the photoconductive layer 62. It will be understood that the transparent conductive layer 66 serves at least three functions. It maintains the target at the potential level of V It increases the efficiency of the system by reflecting a portion of the ultraviolet light toward the photochromic material. It also allows some green probing light to reach the photochromic material.

The system also includes a probing unit which serves as the source of green probing or reading light comprising lamps 70 located externally of a tube envelope 72 and behind a green filter 74. A similar unit comprising lamps 76 behind a red filter 78 is provided for erasing the target 52. Both the probing unit and the erasing unit may also include reflective coatings 80 to maximize the intensity of light directed toward the target 52.

During the writing mode, the electron beam source 54 is turned on so as to irradiate the phosphor layer 60 with a writing electron beam at a selected location. After the writing electron beam penetrates the semitransparent conductive layer 66 and reaches the phosphor layer 60, ultraviolet light is emitted which then darkens the photochromic material 64 at a selected location determined by the deflection fields. During the reading mode, green light is emitted from the probing unit while the photoconductive layer 62 is irradiated by the reading electron beam at a selected location detertion, the current flow resulting from the irradiation of the photoconductive layer by the reading electron beam remains at a low level. This level is detected by reading circuitry 82 associated with the conductive layer 66 and the signal electrode 68 including a resistor 84. On the other hand, when the reading electron beam is irradiating the photoconductive layer 62 at a selected location and the photochromic material 64 is not darkened at that location, a high level of current will be detected. Thus as the reading electron beam is moved across the target 52, locations having darkened photochromic materials 64 will attenuate the probing or reading light causing a reduction in the conductivity in the photoconductive layer. The presence of the reading electron beam at the darkened location will cause a dip in the current through the layer which will appear as a current pulse through the resistor 84. The photoconductive layer 62 will display higher conductivity at cations where the photochromic material is not darkened, thus a difference in pulse height between the two selected locations can be detected. The reading electron beam may be scanned or digitally indexed from location to location on the target.

Erasure of the target 52 may be accomplished by turning the erasing unit on for a period of time sufficient to bleach the photochromic material 64. The transparency of the photoconductive layer 62 and the signal electrode 68 allows a portion of the bleaching light to reach the photochromic material 64.

The previously described systems may utilize a photochromic material such as that disclosed in the following publications which are incorporated herein by reference: GK. Megla, Optical Properties and Applications of Photochromic Glass, Applied Optics, Vol. 5, No. 6, June 1966; and, W.I-l. Armistead and SD. Stookey, Photochromic Silicate Glasses Sensitized By Silver Halides, Science, Vol. 144, April 1964, pp 150-154. A suitable material is also disclosed in U.S. Pat. No. 3,208,860, W.H. Armistead and SD. Stookey inventors, entitled Photochromic Material and Article Made Therefrom" which is also incorporated herein by reference.

Although particular embodiments of the invention have been shown, various modifications may be made without departing from the spirit of the invention as set forth in the appended claims.

What is claimed:

1. A non-viewing information storage and retrieval system comprising:

a target having a light emitting phosphor layer, a photochromic material darkened by light emission from said phosphor layer in response to irradiation by an electron beam and a light sensitive layer for detecting light transmitted through said photochromic material, said photochromic material being located between said phosphor layer and said light sensitive layer;

an electron beam source generating a writing electron beam for irradiating said target, said writing electron beam storing information at said target by darkening said photochromic material in response to light emission from said phosphor layer upon irradiation by said writing electron beam;

a source of reading light for applying reading light to said photochromic material;

a deflection means for moving said writing electron beam across said target so as to store information at different locations on said target; and

a reading means for generating an electrical signal in response to the intensity of said reading light transmitted through said photochromic material at different locations on said target detected by said light sensitive layer.

2. The system of claim 1 wherein said light sensitive layer emits light in response to light transmitted through said photochromic material and said reading means detects light emitted from said light sensitive layer.

3. The system of claim 2 wherein said phosphor layer emits ultraviolet light and said light sensitive layer comprises a phosphor layer emitting red light upon irradiation of ultraviolet light from said ultraviolet light emitting phosphor.

4. The system of claim 3 wherein said source of reading light includes means for operating said electron beam source to generate a reading electron beam of lower intensity than said writing electron beam for irradiating said ultraviolet light emitting phosphor to provide said reading light and wherein said deflection means moves said reading electron beam across said target so as to retrieve information at different locations on said target.

5. The system of claim 4 further comprising:

an additional electron beam source for generating an erasing electron beam directed at said red light emitting phosphor layer, said erasing electron beam bleaching said photochromic material by red light irradiation from said red light emitting phosphor layer; and

a second deflection means for moving said erasing electron beam across said target so as to bleach selected darkened areas of said photochromic material.

6. The system of claim 5 further comprising:

an envelope enclosing said target and said electron beam source, said reading means comprising a photon detector located externally of said envelope.

7. The system of claim 1 further comprising:

another electron beam source for generating a reading electron beam for irradiating said light sensitive layer; said reading light being transmitted through said phosphor layer to said photochromic material; and

said light sensitive layer comprising a photoconductive material responsive to the intensity of reading light transmitted through said photochromic mate rial, said reading means detecting differences in current flow generated by said reading electron beam as said reading electron beam is moved across said target.

8. The system of claim 7 wherein said target comprises signal electrodes, one signal electrode overlying said phosphor layer, said other signal electrode located between said photoconductive layer and said photochromic material, said reading means including circuitry connected to said signal electrodes for determining variations in current flow as said reading electron beam is moved across said target.

9. The system of claim 8 wherein said source for generating said writing electron beam and said source for generating said reading electron beam are positioned in substantially the same plane as said target, and at one edge thereof, said writing electron beam and said reading electron beam having a 45 trajectory with respect to said target at the respective sources thereof.

10. The system of claim 8 further comprising a deflection means generating fields for moving said writing electron beam and said reading electron beam across said target, the same fields deflecting both said writing electron beam and said reading electron beam.

11. An information storage and retrieval system comprising:

a target having a first phosphor layer emitting ultraviolet light in response to irradiation by an electron beam, a second phosphor layer emitting red light in" response to irradiation by an electron beam, a photochromic material darkened by ultraviolet light emitted from said first phosphor layer and bleached by red light emitted from said second phosphor layer, said photochromic material lying between said first phosphor layer and said second phosphor layer, and a first electron beam source for generating a writing electron beam irradiating said first phosphor layer thereby darkening said photochromic material in response to ultraviolet light emitted from said first phosphor layer; a first deflection means for moving said writing electron beam across said target so as to store information by darkening said photochromic material in response to ultraviolet light emission at different locations on said target; source of reading light for applying reading light photochromic material; a reading means for generating a signal in response to variations in said reading light transmitted through said photochromic material at different 10- cations of said target; second electron beam source for generating an erasing electron beam irradiating said second phosphor layer thereby bleaching said photochromic material in response to red light emitted from said red phosphor layer; and second deflection means for moving said erasing electron beam across said target so as to erase information by bleaching said photochromic material in response to red light emission at different locations on said target. 12. The system of claim 11 wherein said source of reading light includes means for operating said first electron beam to generate a reading electron beam irradiating said first phosphor layer, said first phosphor layer emitting ultraviolet reading light in response thereto, said ultraviolet reading light being transmitted through said photochromic material to said second phosphor layer, said second phosphor layer emitting red reading light in response to said ultraviolet reading light, said reading means detecting said red reading light.

13. The system of claim 11 wherein said source of reading light includes means for operating said first electron beam source to generate a reading electron beam irradiating said first phosphor layer to produce ultraviolet photons transmitted through said photochromic material and said second phosphor layer, said ultraviolet photons being detected directly by said reading means.

14. An information storage and retrieval system comprising:

a target having a light emitting phosphor layer, a photochromic material darkened by light emission from said phosphor layer in response to irradiation by an electron beam, and a light sensitive layer, said photochromic material being located between said phosphor layer and said light sensitive layer; writing electron beam source for irradiating said phosphor layer with a writing electron beam to store information in said target by darkening said photochromic material;

a source of reading light for applying reading light to said photochromic material;

reading electron beam source for irradiating said light sensitive layer with a reading electron beam for retrieving information represented by the darkened photochromic material;

a deflection means generating fields for moving said writing electron beam and said reading electron beam across said target so as to selectively store and retrieve information at different locations on said target, said writing electron beam and said reading electron beam being deflected by the same fields; and

reading means for generating a signal in response to reading light transmitted through said photochromic material as said reading electron beam is moved across said target. 

1. A non-viewing information storage and retrieval system comprising: a target having a light emitting phosphor layer, a photochromic material darkened by light emission from said phosphor layer in response to irradiation by an electron beam and a light sensitive layer for detecting light transmitted through said photochromic material, said photochromic material being located between said phosphor layer and said light sensitive layer; an electron beam source generating a writing electron beam for irradiating said target, said writing electron beam storing information at said target by darkening said photochromic material in response to light emission from said phosphor layer upon irradiation by said writing electron beam; a source of reading light for applying reading light to said photochromic material; a deflection means for moving said writing electron beam across said target so as to store information at different locations on said target; and a reading means for generating an electrical signal in response to the intensity of said reading light transmitted through said photochromic material at different locations on said target detected by said light sensitive layer.
 2. The system of claim 1 wherein said light sensitive layer emits light in response to light transmitted through said photochromic material and said reading means detects light emitted from said light sensitive layer.
 3. The system of claim 2 wherein said phosphor layer emits ultraviolet light and said light sensitive layer comprises a phosphor layer emitting red light upon irradiation of ultraviolet light from said ultraviolet light emitting phosphor.
 4. The system of claim 3 wherein said source of reading light includes means for operating said electron beam source to generate a reading electron beam of lower intensity than said writing electron beam for irradiating said ultraviolet light emitting phosphor to provide said reading light and wherein said deflection means moves said reading electron beam across said target so as to retrieve information at different locations on said target.
 5. The system of claim 4 further comprising: an additional electron beam source for generating an erasing electron beam directed at said red light emitting phosphor layer, said erasing electron beam bleaching said photochromic material by red light irradiation from said red light emitting phosphor layer; and a second deflection means for moving said erasing electron beam across said target so as to bleach selected darkened areas of said photochromic material.
 6. The system of claim 5 further comprising: an envelope enclosing said target and said electron beam source, said reading means comprising a photon detector located externally of said envelope.
 7. The system of claim 1 further comprising: another electron beam source for generating a reading electron beam for irradiating said light sensitive layer; said reading light being transmitted through said phosphor layer to said photochromic material; and said light sensitive layer comprising a photoconductive material responsive to the intensity of reading light transmitted through said photochromic material, said reading means detecting differences in current flow generated by said reading electron beam as said reading electron beam is moved across said target.
 8. The system of claim 7 wherein said target comprises signal electrOdes, one signal electrode overlying said phosphor layer, said other signal electrode located between said photoconductive layer and said photochromic material, said reading means including circuitry connected to said signal electrodes for determining variations in current flow as said reading electron beam is moved across said target.
 9. The system of claim 8 wherein said source for generating said writing electron beam and said source for generating said reading electron beam are positioned in substantially the same plane as said target, and at one edge thereof, said writing electron beam and said reading electron beam having a 45* trajectory with respect to said target at the respective sources thereof.
 10. The system of claim 8 further comprising a deflection means generating fields for moving said writing electron beam and said reading electron beam across said target, the same fields deflecting both said writing electron beam and said reading electron beam.
 11. An information storage and retrieval system comprising: a target having a first phosphor layer emitting ultraviolet light in response to irradiation by an electron beam, a second phosphor layer emitting red light in response to irradiation by an electron beam, a photochromic material darkened by ultraviolet light emitted from said first phosphor layer and bleached by red light emitted from said second phosphor layer, said photochromic material lying between said first phosphor layer and said second phosphor layer, and a first electron beam source for generating a writing electron beam irradiating said first phosphor layer thereby darkening said photochromic material in response to ultraviolet light emitted from said first phosphor layer; a first deflection means for moving said writing electron beam across said target so as to store information by darkening said photochromic material in response to ultraviolet light emission at different locations on said target; a source of reading light for applying reading light photochromic material; a reading means for generating a signal in response to variations in said reading light transmitted through said photochromic material at different locations of said target; a second electron beam source for generating an erasing electron beam irradiating said second phosphor layer thereby bleaching said photochromic material in response to red light emitted from said red phosphor layer; and a second deflection means for moving said erasing electron beam across said target so as to erase information by bleaching said photochromic material in response to red light emission at different locations on said target.
 12. The system of claim 11 wherein said source of reading light includes means for operating said first electron beam to generate a reading electron beam irradiating said first phosphor layer, said first phosphor layer emitting ultraviolet reading light in response thereto, said ultraviolet reading light being transmitted through said photochromic material to said second phosphor layer, said second phosphor layer emitting red reading light in response to said ultraviolet reading light, said reading means detecting said red reading light.
 13. The system of claim 11 wherein said source of reading light includes means for operating said first electron beam source to generate a reading electron beam irradiating said first phosphor layer to produce ultraviolet photons transmitted through said photochromic material and said second phosphor layer, said ultraviolet photons being detected directly by said reading means.
 14. An information storage and retrieval system comprising: a target having a light emitting phosphor layer, a photochromic material darkened by light emission from said phosphor layer in response to irradiation by an electron beam, and a light sensitive layer, said photochromic material being located between said phosphor layer and said light sensitive layer; a writing electron beam source for irraDiating said phosphor layer with a writing electron beam to store information in said target by darkening said photochromic material; a source of reading light for applying reading light to said photochromic material; a reading electron beam source for irradiating said light sensitive layer with a reading electron beam for retrieving information represented by the darkened photochromic material; a deflection means generating fields for moving said writing electron beam and said reading electron beam across said target so as to selectively store and retrieve information at different locations on said target, said writing electron beam and said reading electron beam being deflected by the same fields; and a reading means for generating a signal in response to reading light transmitted through said photochromic material as said reading electron beam is moved across said target. 